I have this script

import argparse
parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group()
group.add_argument('-l', '--list', action='store_true', help="show help")
group.add_argument('-u', '--upgrade', action='store_true', help="show help")
parser.parse_args(['--li'])

and this is the unexpected result:

Namespace(list=True, upgrade=False)

And what is your expected result, and why?

I think --li is not the same argument as --list

So I expect that

parser.parse_args(['-l']) or parser.parse_args(['--list'])

should be only two options which should leads to the result

Namespace(list=True, upgrade=False)

argparse allows abbreviations by default so --li matches --list as a prefix match. This is documented at https://docs.python.org/2.7/library/argparse.html#argument-abbreviations-prefix-matching

From python 3.5 allow_abbrev=False can be passed to ArgumentParser to disable this. See also issue10981 and issue14910 that implemented this. https://docs.python.org/3/library/argparse.html#argument-abbreviations-prefix-matching.

Although note that with allow_abbrev=False, -l will conflict with --long, which I'm not sure is a great design.

I'm closing this as not a bug, since it works as documented and there's a workaround (albeit not in 2.7, which is closed to new features).